The present invention relates to a method of adaptively compensating for load changes, particularly in connection with an electric motor drive which comprises a motor, the shaft of the motor being connected to a load having a known moment of inertia; a device, such as an inverter, to control the motor, a torque controller being arranged in connection with the device; and a speed controller arranged to control the speed of the motor.
The present electric motor drives enable a load to be controlled with great precision. For instance, the angular speed or the torque that is acting on the shaft of the motor can be made to keep the desired value quite accurately. Particularly in speed-controlled groups provided with several motors that are interconnected in one way or another, it is extremely important to keep the angular speed of each motor of the group under strict control. Such a speed-controlled group can be a cylinder group of a paper machine, for example. The speed of the rotating rolls of the cylinder group should be controlled such that the speed of the paper web running between the rolls remains the same at each roll.
When a speed-controlled group is subjected to a load change, the speed of the rotating rolls changes from its set value, in which case the speed controller of the motor drive reacts and, eventually, corrects the speed by returning it to the set value. In connection with paper machines, load changes affecting the web speed are caused for instance by cleaning doctor blades of cylinders pressing against the rolls, blades of coating stations pressing against backing rolls, and various closing nips, i.e. rolls that are pressed against each other. When the load of a motor drive increases, for instance when a cleaning doctor is pressed against a roll, the speed of the drive tends to drop. In the opposite situation, in other words when, for example, the cleaning doctor departs from the surface of the roll, the speed of the drive tends to increase. In connection with paper machines, when the paper web runs between the rolls, abrupt uncompensated changes in speed also affect the web tension of the paper in the machine direction.
Previously known attempts to compensate for predictable load changes employ a method of adding an experimentally determined additional command to the torque commands of an electric motor drive at a suitable moment. Such a procedure for compensating for load changes is, however, rather inefficient since it is difficult to determine a suitable additional torque command. Furthermore, load changes vary in magnitude and time of occurrence owing to mechanical wear and changing delays.